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Crop evapotranspiration is calculated using the single crop coefficient approach described in FAO56 (Equation 6). Crop coefficients at various growth stages can be modelled in Source as described in the Irrigation Demand Model Crop Factors SRG entry. Alternatively, Source also offers sufficient flexibility to apply daily crop factors if they are known for a particular crop based on a different source or method.
The effects of soil water stress on crop ET are calculated by multiplying the crop coefficient by the water stress coefficient (Ks). The Ks value is evaluated based on soil water depletion at the start of the time-step.
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Rainfall runoff occurs when rainfall results in soil moisture exceeding saturation or a maximum target pond level for ponded crops.
| Equation 8 |
where:
RainfallRunoff = the amount of rainfall runoff depth for cropping area (m)
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DeepPercolation = amount of deep percolation (m)
Runoff occurs from both cropped and fallow areas. In addition, runoff occurs from an Undeveloped Area if configured.
Deep Percolation, Irrigation runoff, district losses and escapes
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The target depletion is a user specified input and can be specified as either a pattern or as an expression. The targets are defined in terms of soil water depletion. They are positive numbers for non ponded crops where the soil is typically in deficit. For ponded crops, the soil water depletion is negative. Thus targets for ponded crops are entered as negatives.
Two Three target depletions can be specified
- Regulated Target. This is the target at which Irrigators water and attempts to maintain soil depletion; and
- An Opportunistic Target. This target is used to generate opportunistic requests. For ponded crops, it also represents the maximum pond level prior to runoff. By default the opportunistic target is disabled and no opportunistic requirement is generated.
- A Refill Trigger - May allow better representation of irrigation scheduling for individual properties as irrigation does not commence until the forecasted soil depletion reaches the specified refill trigger
The behaviour of the first two target levels is illustrated in the following two Figures for a ponded and non ponded crop.
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Figure 8. Ponded crop pond level maintenance
If the Refill Trigger is activated, and a soil depletion value greater than the Regulated Target defined, orders will not occur until the forecasted soil depletion reaches the specified trigger. Orders are then placed at the current timestep, and if necessary at subsequent timesteps, until the forecasted soil moisture depletion is less than or equal to the Regulated Target. This allows for the possibility that restrictions such as pump capacity mean that it is not possible to meet the target in one timestep.
Forecasting depletion
The Irrigator model calculates a list of regulated and opportunistic irrigation requirements between now and the maximum travel time. This requires that soil depletion, as defined by the soil water balance (Equation 1), is forecasted from the current time-step to the maximum travel time. This requires forecasts of evapotranspiration, rainfall, deep percolation, runoff and water that has been previously ordered.
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